At the start of the study in 2004, the women were 60 or older and responded to surveys on their use of antibiotics when they were young (ages 20 to 39), middle-aged (ages 40 to 59) and after age 60. The women continued filling out questionnaires every two years.

During a follow-up of about eight years, researchers noted that 1,056 of the participants developed heart disease. Ultimately, duration of antibiotic use in middle and older adulthood (not young adulthood) showed significant associations with heart disease later in life.

After adjusting for other factors such as major diseases, race, sex, age and diet, researchers found that compared with women who didn’t use antibiotics in late adulthood, those who took them for two months or longer were 32% more likely to develop heart disease. And women who took antibiotics for two months or longer in middle age had a 28% increased risk.

“As these women grew older they were more likely to need more antibiotics, and sometimes for longer periods of time, which suggests a cumulative effect may be the reason for the stronger link in older age between antibiotic use and cardiovascular disease,” study author Yoriko Heianza of Tulane University said in a statement.

According to senior author Lu Qi, a professor at Harvard University, antibiotic use is known to alter the balance of gut microorganisms, which can in turn lead to “inflammation and narrowing of the blood vessels, stroke and heart disease,” he told the Independent.